S/N detection strength as a function of SYSREM iterations for the signal of CO, at nodding positions A (blue colour) and B (orange colour). The top panel shows the signal strength for the real data, and the bottom panel shows the strength of an injected signal using the differential ΔCCF according to the method described in Sect. 3.3. The diamond shapes indicate the SYSREM iteration with the strongest detection of the injected signal.